Raspberry Pi 3 + Qt5.9 = QML application => Texts are not shown

I developed a QML application with Qt5.9.
I build this application for Ubuntu and for Raspberry Pi 3.
On Ubuntu everything is fine.
On Raspi 3 everything was fine. Now I have the problem that all the texts of labels, textfields, tables, ... are not shown.
The images, the borders, the colors, the graphs, ... are shown normally.
But I get no error messages while compilation and deyployment.
Does anyone know this problem and the reason for this problem?